MRAM (magnetic random-access memory) tunnel junctions used in magnetic memory arrays must operate in a tight window of switching fields in order reliably to write data. Unfortunately, the magnetic fields in the operating environment of the chip are not under the control of the designer. In addition, temperature fluctuations can also affect the performance of magnetic memory cells by causing changes in their astroid parameters and similar problems. Therefore, tolerance for variances in operating conditions must be designed into the chips to allow the chips to operate in the fluctuating environment. This over designing of the chips increases the cost and size of the memory array.
Shielding of MRAM devices is possible using various arrangements of high permeability magnetic sheets in proximity to the device. However, due to a low shield factor (given by the ratio of external field to the field felt by the device) or an external field that exceeds the saturation field of the shield material, the MRAM device can be exposed to significant magnetic fields due to dynamic environmental sources. Shielding will also be ineffective if the fields imposed on the package are below the coercive field of the shielding as well. In addition, shielding magnetic memory arrays with sheets of high magnetic susceptibility materials in various geometries is expensive. Furthermore, in order to achieve a high shield factor (i.e. the ratio of the external field to the field at the MRAM chip) more sophisticated and, therefore, expensive geometries must be employed. Nevertheless, it would be desirable to reduce these fields further, or else reduce the effect of these fields on the device, particularly during the write operation when fluctuations in magnetic fields may cause a critical write error.
Typically, the magnetic fields required to switch the MRAM device are in the range of 30 to 70 Oersted (Oe), and the standard deviation in the switching fields of the devices, or the applied fields, either intentionally or from external magnetic noise sources, must be held to less than 2 or 3 Oe. If this requirement is not met, the errors in the write process due to either writing half-selected cells or failing to write the intended target cells will rise to unacceptable levels. Unfortunately, much of this field spread budget is consumed in fabrication tolerances and so the external field effects should be minimized as much as possible, preferably below 0.2 Oe or so.